As Hurricane Sandy nears the eastern third of the country, thousands of flights and trains have been canceled, while New York City's rapid transit system, the largest in the world, has begun to be shut down.

Gov. Andrew Cuomo said the subway would be suspended at 7 p.m., while bus and railroad services would be suspended beginning at 9 p.m. ET. He said he hoped the closure would keep New York-area residents from being "up and about."

"[Trains are] not intended to be submerged, and we want people to stay at home," he said.

Approximately 15 million people in New York City, its suburbs, New Jersey and Connecticut rely on the transit system.

Amtrak is canceling its East Coast service on Monday, while New Jersey Gov. Chris Christie said the New Jersey Transit system would be suspended beginning at 12:01 a.m.

On Monday, 5,559 flights have been cancelled and 613 cancelled for Tuesday, Flight Aware reported. The most affected airport today was Newark Liberty International, with 305 cancellations.

People scheduled to fly to or from the eastern third of the country are encouraged to check their flight status

Every airline has issued a flexible travel policy that allows customers to make changes to their reservations with no penalty. Check your individual carrier's homepage for details.

United Airlines, with a hub in Newark, N.J., has been the most aggressive so far in getting ahead of the storm. According to FlightAware, the airline has cancelled more than 300 flights today.

The airline told ABCNews the majority of those flights are scheduled for Sunday evening.

"United is working to operate a normal schedule to the East Coast through the late afternoon hours. We have selectively canceled flights beginning this evening through tomorrow to many airports in the northeast and mid-Atlantic states," Rahsaan Johnson, an airline spokesperson, said in an email.

"Travelers who are booked on a flight to or from a city in the storm's path this evening or Monday should assume it is delayed or canceled and should verify flight status at united.com," Johnson said. "Customers whose flights are canceled or delayed more than two hours are eligible for a refund."

Some hotels are offering discounts to travelers stranded by the storm. Kimpton hotels in New York City and Boston offer 20 percent-off rates. Use promotion code SANDY.

In Washington, D.C., the Ritz-Carlton, Georgetown offers a special nightly rate of $229 including free parking and Wi-Fi.

Hurricane Sandy is expected to bring life-threatening storm surge flooding to the Mid-Atlantic coast and could wreak havoc for days across 800 miles of the United States, affecting tens of millions of people in the eastern third of the country.

Sandy will meet up with cold front coming from the northwest and a high pressure system from Greenland, fueling it with enough energy to make it more powerful than the "Perfect Storm," some meteorologists say.

"This storm that is going to be impacting the mid-Atlantic and parts of the Northeast ... is going to be destructive, historic, and unfortunately life threatening," AccuWeather's Bernie Rayno said.